In most cases, the larvae don't eat for a few days after they hatch, although in some cases they hatch a little bit late and ready to start eating. Add just a few of the water fleas (Daphnia) for the first week or so. Once you see that they are eating, add larger amounts of water fleas.
I have a piece of screen that I use to separate large/small daphnia. If the daphnia look a bit too large for the axies, you might want to try something like this to sort out just the smallest daphnia for starters.
What kind of container are you keeping them in? And how many hatchlings do you have?
There are some good articles online about this whole process. Have you found them? If not, we can post links.